1. Field of the Invention
The invention relates to a method of obtaining a structure on a semiconductor wafer by etching through a dielectric layer defined by a mask using a plasma.
2. Description of the related art
Plasma-etching processes are commonly used in the fabrication of semiconductor devices. Generally, photoresist material forms feature patterns on the surface of the wafer to be etched, and features are then etched into the wafer by exposing the wafer to a particular type of etching gas. One of the challenges faced in plasma etching is the ever-increasing aspect ratio needed to meet design requirements, especially for ultra-high density structures. When etching features on semiconductor wafers, the aspect ratio of an etched feature is defined as the ratio between the feature's depth (d) and the feature's width (w) or diameter. As more features are packed on a single piece of wafer to create higher density structures, the width (w) or diameter of each individual feature necessarily decreases, while the depth of the features remains unchanged or increases. Thus, the aspect ratio of each individual feature increases as the device feature shrinks.
A difficulty during ultra-high aspect ratio (UHAR) etching is twisting and or distortion, which is generally defined as deviations of location, orientation, shape, and size near the bottom of a feature from the pattern defined by the mask on the top of the feature. When the aspect ratio of a feature reaches a certain threshold, while the feature's width is very small, twisting occurs, particularly near the bottom of the feature. In addition, such UHAR etches are subjected to aspect ratio dependent etching (ARDE). These difficulties are further disclosed in U.S. patent application Ser. No. 11/562,335, entitled “REDUCING TWISTING IN ULTRA-HIGH ASPECT RATIO DIELECTRIC ETCH,” to Ji et al., filed Nov. 21, 2006, which is incorporated by reference for all purposes.